While standard HTML files are fine for storing pages, it is very useful to be able to create some content dynamically. For example, to add a footer or header to all files, or to insert document information such as last modified times automatically. This can be done with CGI, but that can be complex and requires programming or scripting skills. For simple dynamic documents there is an alternative: server-side-includes (SSI).

SSI lets you embed a number of special 'commands' into the HTML itself. When the server reads an SSI document, it looks for these commands and performs the necessary action. For example, there is an SSI command which inserts the document's last modification time. When the server reads a file with this command, it replaces the command with the appropriate time.

SSI Implementation: All Webtechs Internet hosted sites come with SSI pre–installed. To make an HTML page using SSI name the page .shtml This signals the server to look for embedded SSI tags described below. If the page is not named .shtml the server will ignore the tags as comments.

SSI Commands: All SSI commands are stored within the HTML in HTML comments. For a list of commands visit the NCSA tutorial site. A typical SSI command looks like this:

<!--#flastmod file="this.html" -->

In this case the command is flastmod, which means output the last modified time of the file given. The arguments specify the file "this.html" (which might be the name of the file containing this command). The whole of the command text, including the comment marker <!-- and --> will be replaced with the result of this command. In general, all commands take the format:

<!--#command arg1="value1 arg2="value2 ... -->

where arg1, arg2, etc are the names of the arguments and value1, value2 etc are the values of those arguments. In the flastmod example, the argument is 'file' and it's value is 'this.html'. Often commands can take different argument names. For example, flastmod can be given a URL with the argument virtual, to get the last modified time from the server. For example, use:

<!--#flastmod virtual="/" -->

to get the last modification time of the home page on the server (this is useful if the page being accessed might have a different file name, for instance).

Besides flastmod, there are SSI commands which get the size of a file or URL, the contents of a variable (passed in by the server), the contents of another file or URL, or the result of running a local file. These are documented in the NCSA tutorial on server side includes.

When SSI commands are executed, a number of 'environment variables' are set. This include the CGI variables (REMOTE_HOST etc), and some more, such as DOCUMENT_NAME and LAST_MODIFIED. These can be output with the echo command (so a better way of getting the last modification time of the current file would be <!--#echo var="LAST_MODIFIED" -->).

Examples: Here are some examples of using SSI

Displaying document information: The following code puts the document modification time on the page:

Last modified: <!--#echo var="LAST_MODIFIED" -->

Adding a footer to many documents: Add the following text to the bottom of each of the documents:

<!--#include file="footer.html" -->

Hide links from external users: Use the if command and the REMOTE_ADDR CGI variable to see if the user is in the local domain:

<!--#if expr="$REMOTE_ADDR = /^1.2.3./" -->
<a href="internal-documents.html">Internal Documents</a>
<!--#endif -->

(Where 1.2.3 is the IP address prefix of the local domain).